The synonym of “Prolong” is:

The correct answer is Extend.

• The word “Prolong” means to make something last for a longer period of time or to extend its duration.
• Option A (Extend) is a direct synonym of “Prolong” as it also means to make something longer or to continue for a longer time.
• Option B (Shorten) is an antonym of “Prolong,” meaning to make something shorter.
• Option C (Cancel) means to call off or annul something, which is unrelated to the duration of an event.
• Option D (End) means to bring something to a conclusion or stop it, which is the opposite of prolonging it.